Core drilling is a widely employed method for inspecting earth formations deep below the surface. The typical method involves drilling a borehole on the order of a few inches in diameter, and obtaining one or more core samples. The cores are stored in the coring device and may be studied after the device is removed from below the surface.
One popular type of drill bit used in core drilling is a diamond bit, which includes a matrix to which is affixed a plurality of diamonds. The bit is rotated at high speeds and is advanced downwardly in order to create a cylindrical borehole. The drill bit is typically annular to define a central opening. Thus, as the drill bit is advanced through the earth, a portion of the earth is forced through the central opening. In this manner, a core sample is obtained and stored for later inspection.
While diamond drill bits are efficient when used properly, there are a number of shortcomings associated with those bits as well. When using diamond drill bits, the weight on the bit is of critical importance. If too little weight is applied to a bit, then the rock in contact with the rotating bit tends to polish the diamonds, such that they become much less efficient in cutting through the rock. On the other hand, if too much weight is applied to the bit, diamonds tend to be stripped from the matrix, thereby destroying the bit. In either event, the operator must replace the bit, which is not only expensive, but can be very time-consuming as the drill string must be raised and dismantled piece-by-piece before access can be had to the bit. In the case of a drill string hundreds of feet long, with each drill string segment being 10 to 20 feet long, such a procedure is time-consuming and extremely inefficient.
Many prior art systems simply rely on the operators' expertise in order to prevent damage to the drill bits. Those systems include support/feed hydraulics to control advancement of the drill bit, and also incorporate pressure gauges that monitor the pressure in the hydraulic system. Thus, the operator must monitor the pressure gauge and use that information to estimate the actual weight applied to the bit. To further complicate matters, these prior art systems operate in two modes, a "pull down" mode and a "hold back" mode. In the "pull down" mode, the hydraulic system actually forces the bit downwardly through the earth. In the "hold back" mode, the hydraulic system takes weight off of the drill string and thus the drill bit. In the "pull down" mode, the weight on the drill bit is determined by reading the pressure gauge in a straightforward manner. However, in the "hold back" mode, the pressure gauge must be read in reverse to estimate the weight on the drill bit. Thus, it is apparent that such systems require an experienced, attentive operator who can perform these estimations virtually instantaneously in his or her head. Any operator error or a momentary lapse of attention can result in destruction of the drill bit which, as described above, results in a costly and time-consuming replacement procedure.
A feedback control loop for a core drilling system is disclosed in U.S. Pat. No. 4,714,119 to Hebert et al. The system includes a core drilling mechanism that can be rotated from a vertical to a horizontal position in order to obtain a core sample from a side wall of a pre-drilled borehole. The system includes a feedback loop that controls the weight on the bit. The feedback loop operates in response to the back pressure on the coring motor to manipulate a needle valve in the hydraulic circuit. Thus, as resisting torque increases, the back pressure increases. In response, the feedback controller slows the forward movement of the coring bit. This system is not concerned with or suitable for use in solving the problem of the entire string weight being applied to a vertically moving drill bit. When a drill bit stops penetrating or slows down considerably, it can be due to a mismatch between the bit and the rock, or due to a dull bit. Neither of these scenarios necessarily result in an increase in the back pressure in the motor circuit. Thus, this prior art system would be wholly ineffective in such situations and would not prevent drill bit damage. Furthermore, this system does not monitor the weight on the bit, but simply monitors whether the head resists rotation, which could happen if, for example, the drilled hole were to collapse. This is quite possible, especially in a horizontal drill hole. Thus, this prior art system addresses different problems and is not suitable for use in solving the problems addressed by the present invention.
A number of prior art systems used in the oil drilling art include feedback systems for controlling weight-on-bit by slowing down, or stopping, the penetration of the drill bit. Examples are U.S. Pat. No. 4,875,530 to Frink et al. and U.S. Pat. No. 5,474,142 to Bowden. These references fail to provide any means for controlling the penetration rate, aside from reducing or zeroing out the penetration rate in the event the weight-on-bit exceeds the preset limit. Thus, these references do not provide a penetration rate feedback control, and are clearly not concerned with drilling at an optimal penetration rate.
Diamond core drilling typically involves relatively light-weight tubing for the drill string, unlike oil well drills, auger drills, rotary percussive drills, and the like, which use much heavier-weight tubing. Thus, a significant concern in the case of diamond core drilling is that the drill string will be subjected to excessive torque loads and will twist off. Often, these torque loads are reached well before the drill bit is subjected to the maximum weight-on-bit that it can handle.
Accordingly, it will be apparent to those skilled in the art that there continues to be a need for a control system for automatically controlling the weight applied to a core drill bit, the torque load applied to the drill string, and the penetration rate of the drill bit, and for maintaining all three within preset ranges. Furthermore, there exists a need for such a control system that simultaneously prevents both the drill bit and drill string from being damaged and optimizes the efficiency of the drilling system. The present invention addresses these needs and others.